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Discussion Starter · #1 ·
Hi everybody, this is my first message in this great forum :) I've been lurking for awhile, reading through a whole bunch of threads, and I'm very impressed with the quality of some posts :)


Since I've noticed there are many people here with great minds, I'm taking this opportunity to ask a question that might be rather silly, but I've never heard a good answer to it.. I'd be happy if you could explain this so even I understand it ;)


Ok, CD players. Since data is stored digitally onto the CD, playing this back would entail reading 1's and 0's off the disc. What I simply don't get it, how can there be any difference whatsoever between various CD transports :confused:


Sure, when the data is off the disc, it's up to the DAC to convert it to sound, and DACs do differ wildly in performance - that we know. But the actual transport.. How could any transport fail to read the digital data off the disc? For all I know, any CD-ROM transport for $30 would succeed in doing this 100 times out of 100, unless it's broken.


I guess the same question could apply to digital cables. How on Earth can one cable be better than another, if all it does is transmit 1's and 0's? Think of the havoc this would make in a computer.. It just cannot be!


I'd be happy if anyone could fill me in here; this is really getting on my nerves!


Cheers,

Peo
 

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i'll don my nomex suit, but here's an answer.


the only rational, obvjective, mathematically-supportable argument that i've ever heard anybody can make on this point is "jitter", which is basically the data being dumped into a bitstream with incorrect timing. this matters because it's reading a continuous stream of information for 74 minutes (or whatever). with a computer, you ask it to open up LetterToAuntMartha.doc and that's 17KB and if it has to read the disc 15 times before it "gets it right" that's still not long enough for you to notice. but with music, it has less leeway because it's pulling a continuous stream of data (this is oversimplifying - there is in fat some time available, but the point remains the same).


here's a totally random site (pulled via google) explaining jitter in more detail (there are hundreds of others): http://www.jitter.de/english/engc_navfr.html


so as long as your transport can pull the bits off successfully and with proper timing there's no difference. 1010101 = 1010101 = 1010101 every day of the week and twice on sundays. if one transport pulls 1010101 and another pulls 1001010 then one of the transports is just plain WRONG. it's not better or worse - it's just WRONG (don't buy that one!).


many transports will re-clock the bitstream. so it says "i know how the bits read from the disc should be spaced to fit the sample rate and word length" so it reads as fast as it can into a buffer and then it's hardware and/or software reorders that stream of bits to be perfectly timed for transport to wherever they're going.


in theory, any time you put these bits on top of some analog transmission medium (whether that be optical cable or copper cable) you can introduce jitter.


some systems reclock at the receiving end (independent of any reclocking at the sending end). with this sort of device, you should be able to feed it via a $17 computer cdrom drive or via a $20,000 Meridian 800 and you will get the exact same series of ones and zeros, with zero jitter and they will have to mathematically sound exactly the same.


if you can hear a difference between them, then the transport is doing something other than pulling ones and zeroes and passing them along. and many transports could and are doing something other than pulling ones and zeroes and passing them along.


DACs are of course an entirely different ball of wax, as you point out.


my two pennies.


doody.


PS: oh and the cable thing is bunk. either the cable is made of some material electrically capable of meeting the design principles (resistance, etc.) or it isn't. if it is, it should be able to get the ones and zeroes from point A to point B. so there may be a difference between a $0.50 1M cable and a $5.00 1M cable, but there damn well shouldn't be any difference between a $50.00 1M cable and a $5,000 1M cable.
 

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I'll take a stab at simplifying it a bit.


It's all about "jitter."


Jitter refers to the timing of the datastream of 1's and 0's. Let't think of a simplistic example to make this easier... a simple square wave. The data on some imaginary (and crude) format is being read and sent to the DAC at one sample per second. This crude data format has only two possible values for any one sample; one and zero. Thus, the data rate is also one bit per second.


For simplicity, imagine that the DAC responds instantly to the digital input. As soon as a 1 hits the DAC, it outputs full power. As soon as it gets a 0, it shuts off.


The important piece of the puzzle here is the timing of those 1's and 0's getting to the DAC. If they arrived perfectly once every second, then a square wave would have exactly one second long peaks. One second on, one second off.


Now, imagine that the CD transport is really, really, really crappy. Instead of sending one bit per second, it sends on average one per second, with a normal variation of +/- 1/4 second. Some bits arrive too early, some too late. The square wave now has a variation in peak lengths. Some are half a second, most around a second, some a second and a half in duration.


Were that a sine wave instead of a square wave, it would show up as a variation in frequency. Thus, the basis upon which it is claimed that some CD transports (and cables) are different is that their levels of output (or transmitted in the case of cables) jitter are different.


The real world is much more complex. CD's send 44,100 samples per second for each channel. Each sample has a possible 65,536 values (amplitudes). Further complicating things is the way CD audio is stored... as pulse code modulated information (PCM). Instead of a stream of "values" coming out 44,100 times per second each having a value of 1, 2, 3... 65,536 or whatever, you have binary 1's and 0's coming out at a much faster rate. A buffer in the DAC (or before actually) stores these 1's and 0's until enough are there to code for a single sample value. It is then integrated by the DAC (simplifying things again).


In an ideal world, that buffer would be able to completely isolate the rate at which it receives data from the transport and cable from the rate at which it sends out the data to the DAC. Unfortunately, in most all cases the output clock to the DAC essentially has to be tied to the timing of the incoming data from the transport. In a sense, the clock of the transport sets the accuracy of timing for the whole system. If the transport's clock has a lot of variability (or a cable introduces a lot of variability), then the samples reaching the DAC might have some variation in their timing (in other words, a time-error which is the difference in when they were supposed to reach the DAC and when they actually get there). That variation can, in theory, cause audible differences.


It gets even more complicated when you realize that the very spacing of the pits and lands etched onto the CD when it was produced can have variability in data timing. Perhaps the burner wasn't consistent or the master press had a variability made into it. Since the rate at which those pits are read affects the rate at which data is sent to the buffer, and hence the DAC, "recorded in" jitter might also be something to take note of.


Computers are a bit different, in that data is routinely held in large buffers in each destination they go to before they are actually used for processing. Computers were designed from the ground up to work on this buffered principle, and deal quite well with it. As an aside, a CD that has been "ripped" and stored on your hard drive, and played back later, is in many ways the perfect CD transport. You have isolated or removed all jitter (timing variability) that might have been recorded in and all jitter related to the reading of the CD by your CD-ROM. It is stored on your hard drive in a completely unclocked format, and is for all practical purposes completely reclocked by the output device in the computer (sound card). The only source of jitter is in the master crystal of the sound card, and good professional or prosumer level cards have respectably low jitter levels.





All that being said, it is still very debated whether jitter is audible, or how much jitter it takes to be audible, or how effective "reclocking" devices are, or how much jitter variation different cables introduces, or whether coax or plastic or glass gives the best real world results, or.... Suffice it to say that the amount of jitter induced distortion in your system will likely be well under the storm of distortion created by analog devices. For all practical purposes, that $30 transport will be nearly impossible to distinguish from a $3000 transport, assuming a good DAC with relatively good buffer isolation characteristics is used. If you have spent tens of thousands on your audio system, and are looking for the next place to chase the unicorn or perfect sound, start looking for expensive transports. Otherwise, spend the money on speakers and room treatment, where the effects will be audible.
 

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Discussion Starter · #4 ·
Thanks for your excellent replies! :)


Yeah, I've heard of jitter, but never really paid attention to it. I've lived in a world where I assumed that everybody knew how to buffer data before feeding it to something as time-sensitive as a DAC. Apparently this still isn't the case!


As you state Bigus, the optimal way is to store it magnetically (or why not in a RAM buffer in the transport/DAC) and then feed the DAC according to an as-perfect clock as possible. That shouldn't be rocket science, but well.. Manufacturers wouldn't make any money on CD players if it wasn't, eh ;)


Cheers,

Peo
 

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The biggest problem is buffer under-run or over-run. CD data comes out at quite a high rate. Something like, what, 1.4Mb/sec (176 kBytes/sec)?


Suppose you had a complete isolation buffer that perfectly reclocked incoming data. If the transport clock ran .1% too fast, you would have an excess of nearly a full megabyte of data by the end of the disc (if it played straight through, something you have to plan for). On the other hand, if it were running .1% too slow, you would have to begin each track by reading a megabyte or so ahead to make sure you didn't run out of data in the buffer.


It would seem like adding a 1MB ram or flash ram module to a DAC wouldn't be very expensive, but my guess is that the complexity of making it work properly is far more critical than the cost. Some of the better DAC's do essentially just that though.


I think a better explanation would be that a well designed PLL (phase locked loop - the thing that creates the output timing clock to the DAC based on the incoming timing clock from the transport) has fairly good jitter rejection, and added to the debatable effect of low jitter levels on audibile changes to the sound, most companies feel that is "good enough."
 

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Discussion Starter · #6 ·
Mm, yes, I understand that. A small 1MB or even 8MB buffer would add little to the overall cost however.. It will hurt your average $100-200 player, but hardly any of the more serious players. I paid $6000 for my player, and if it had a $20 flash-chip inside it, I don't think it'd cost me more :)


Seriously, I think I'm overly negative towards CD players in general. Seems to me that the media and the way of playing it have way too many problems to be practical. I'd vote for bringing the data off-disc before making any attempts to play it, e.g. heavy buffering or why not complete buffering, e.g. store the whole track in a buffer before playing? It's quick enough to rip one track, and memory chips are inexpensive...


Ah well, I'll move to computer-aided playback I think :)
 

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it would seem that the vast majority of problems with regards to playing CD's has been largely overcome. however if you're going to computer playback, it does make that 6K expenditure seem a bit on the high side. CD players though do differ in their ability to handle errors and what not. For those interested, a CD test disc to evaluate players in the comforts of your home is available at http://www.digital-recordings.com/audiocd/audio.html
 

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Discussion Starter · #8 ·
I agree with you there Chu, that today's CD-players are really good. They sure are. I currently own the MF Nu-Vista 3D CD-player, and I rate it VERY highly. My reasons for moving to computer-based playback is not due to quality, it's a matter of comfort.


It's just that I'm interested in the theory of playback and a good discussion :)


Thanks everybody for your nice replies!
 

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I think the theory is interesting also however, to my mind, it rises in importance and significance to the enduser when it's tempered by issues of audibility that've been established. I'm sure your unit is a very attractive one and to some extent, it's regrettable that there aren't more well produced CD's out there.

Not knowing what sort of PC you're using out there Peo, were you aware there is software out there (Apple only I think) that'll take your CD tracks and give them that 'vinyl' sound?
 

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Discussion Starter · #10 ·
Sounds like a cool piece of software! Unfortunately I'm a Windows user and have no Macs at home.. In any case, if anyone knows more about this software I'd be happy to hear about it :)


And yes, CDs aren't generally that well-produced. My vinyl collection is much better in that respect, even though there are lots of crap in that arena as well. But I find it funny how my $1000 Rega P25 can do much better with 25 year old vinyl than a CD player costing six times as much does with a brand new CD. It happens to me all the time!!
 

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I don't know about that, but at least the album jackets were nice looking!
 

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I can only speack out of my own personel experience and A/B Testing.


I am Software developer and I was very skeptic regarding D Trasnport qualities. Even after I read lods of papers about jitters, I was convinced that a a good buffer in the Pre that resync the signal right before re routing to the DAC should make CD Transport sound quality a non issue.


It took me 20 min of quiet listening to change my mind completly.


I own a Tag AV32R Pre/Pro feeding Tag Amp and N802 Speackers. Its is a very revealing system.


Feed the Pre with:

1-HTPC Revo Card

2-Tag CD20R

3-Marantz 8300

4-Nakamichi Av10


and I noticed a difference in sound during every trial!

Weirdly though, The marantz nd Nakamichi were close in performance. byt The Tag clearly outperform them in quality. Much warmer and clearer mids. Needless to say, the Tag cost much more $$$.


________

Sam.
 

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nobody is arguing that some transports do indeed sound different than others.


the assertion is that there should be zero differences if all they're doing is pulling ones and zeroes and slapping them on a wire. as such, those players that do sound "different" are affecting the bitstream in a way you're simply not aware of.


doody.
 

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Hi,

While reading the prvious threads it makes me ask myself how can you really evaluate a cd transport unless you use headphones directly to the player or the player can somehow be cnnected to speakers.


i look at my situation and even though i'm very pleased with what comes out of my speakers, i don't know to what component to give the credit.


My AV reciever has all the latest bells and whisltes to massage the sound once it leaves my transports which are dvd-a and sacd players (both entry level players). With regard to cd's, since WindowsMedia does HDCD decoding, I have recorded these discs to my hddrve and again though an optical cable from my soundcard to my AV reciever which in turn does it's stuff and sends the result to the speakers.


I read review in magazines evaluating DAC's which are being fed signals from a cd transport. so what is really being evaluated here the trnsport or the DAC or both


I think the industry has come far enough in the last 20 years to producean inexpensive transport with little or no jitter, if not theres the buffer in computers.


It brings to mind my first dvdplayer AV recievr purchase. The salesman explained to me that if dolby digital and dts was being decoded in the reciever i didn't have to pay extra to have it in the player.


If someone could give me in laymans terms an argument for a $10,000 cd transport i'd love to hear it. because once the signal leaves the player what componet's responsibility does it become?


Peter m.
 

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It is not trivial to take the digital data from a compact disc and produce the correct sound from it. You can't just send it out a simple digital-to-analog conversion circuit and be done with it. There is a certain amount of processing that must be done in order to properly reproduce the signal---there are many ways to do it, but it must be done.


So DACs (both inboard and outboard), upsamplers, and digital receivers really do need some serious design effort to produce the right sound. I would say that credit must be given in this order:

1) to the speakers (of course :));

2) to the DAC (including any upsampling and digital filtering);

3) to amplification; and

4) to the transport->DAC connection (i.e., jitter).


Bottom line, a CD/DVD player used only as a transport---i.e., with no upsampling or other digital processing performed on the data---then jitter is the only potential source of distortion that can arise from that component. Any other distortion or changes in sound quality must come from other components.
 

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So Mike,

If i understand you correctly, a high quality internal dac in the transport could be overridden (screwedup ) by a poor downstream unit and vice versa.


To get back to my "theoretical" high quality transport with a builtin excellent DAC the desire would be to have downstream equipment strictly do passthru and amplification.


When buying equipment, (lets assume the speakers are excellent) It is most important to have the quality in the unit that is doing the processing just prior to the signal hitting the speakers. Is that correct?


Peter m.
 

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Quote:
is most important to have the quality in the unit that is doing the processing just prior to the signal hitting the speakers.
Peter, I think that's hard to say. After all, there's a "garbage in/garbage out" factor too. In other words, if your source device is lousy, it doesn't matter how good the subsequent processing and amplification steps are. And I also think that quality amplification is a bit easier to obtain than quality digital-to-analog conversion.


Personally, I think I would focus first on the component that includes the DAC---whether it is built into the transport, or contained in an outboard processor or receiver.
 

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Thanks Mike,

One of the reasons I'm so interested in this is that i presently have what would be called by Audiophiles entry level dvd-a and sacd players.


However my AV reciever and speakers are quite the opposite and i find the sound coming though the speakers extremly detailed and excellent overall and i'm asking myself what could be an expensive question.


The Denon has been compared to all but the highest end separates in several professional reviews and if it's doing the bulk of the work after the sound leaves the transports, would buying higher quality transports be overkill, or worse still would they be rendered moot if the reciever takes over.


Peter m.
 

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To quote myself from another thread


"my local dealer says that no SACD or DVD capable player or transport will be as good as redbook CD only transports or players because of the difference in motor speed. He insists that electric motors cannot be made to be stable rotating at more than one rotational speed. Since redbook CD rotates much slower than SACD or DVD, only a transport/player optimized for that speed will perform best for that media. "


This is an intuitively appealing argument because I can understand how stable rotational speed of the CD would be critical in reading a time sensitive data stream from that disc. On the other hand, how stable is stable enough? Can a motor really only perform well at one speed? As someone else said, maybe my dealer is thinking that CD transports are more like turntables than they really are.


Can minute variations in rotational speed produce jitter? If not, what other artifacts might result from inconstant speed? Not wow and flutter thats for sure ;)


Mark H
 

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Peter-


I can't speak to your pre/pro, but generally the DACs in Pre/pros are not as good as the DACs in good stand alone CDPs. FWIW, I compared the analog vs. digital output from my old Meridian 506.20 to my Ref30 and found the analog output from the Meridian was much more smooth and detailed.


To build a good DAC you HAVE to have a very clean power system. Receivers are notorious for lousy noisy power systems.


When you're thinking about digital signals people generally picture square waves. In reality, there is a noninfinite slope to the voltage in the transition zones. The DAC reads a 0 or 1 in this signal by measuring the voltage of the signal compared to a "fixed" voltage- generally ground. If the ground supply is connected to lots of circuits and not properly isolated from EMI the ground voltage will vary and thus you will have more jitter. This is additive with whatever source jitter you have and results in more harshness and edginess to the sound. This is frequently the case in pre/pros. The best DAC chips ever created can't overcome limitations in their power supply.
 
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